The disclosure relates to a printing system having a linear diffractive spatial light modulator (LDSLM) assembly that diffracts light from a laser source according to or under the influence of an applied electric field applied to the LDSLM assembly. In one embodiment, the LDSLM assembly includes a linear array of diffractive MEMS elements. For example, each of the diffractive MEMS elements can include a number of deformable ribbons having a light reflective planar surface. Preferably, the linear array of diffractive MEMS elements including the ribbons and drive electronics are integrally formed on a single substrate. In other embodiments, the LDSM assembly can include two or more linear arrays of diffractive MEMS elements, and the laser source can include an array of multiple lasers or laser emitters.
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10. A method of printing comprising:
emitting a light beam;
focusing the light beam onto a linear diffractive spatial light modulator;
diffracting the light beam by controllably diffractive elements of the linear diffractive spatial light modulator according to an applied field at each element; and
imaging the light beam from the spatial light modulator to a substantially linear portion of a photosensitive surface at an image plane, wherein said imaging includes both forward and inverse fourier transformations of light; and
a drum covered by the photosensitive surface, wherein the drum is configured to rotate the photosensitive surface such that the substantially linear portion imaged by the light beam is scanned across a two-dimensional area of the photosensitive surface,
wherein the emitted light beam comprises beam components emitted by a plurality of laser emitters, and the focusing of the light beam onto the spatial light modulator comprises magnifying each beam component and bending the beam components to different incident angles.
9. A printing system comprising:
a laser source to generate a light beam;
a linear diffractive spatial light modulator assembly to diffract light from the laser source according to an applied electric field;
illumination optics for focusing the light beam onto the spatial light modulator assembly;
an image plane having a photosensitive surface;
imaging optics disposed in a light path between the spatial light modulator assembly and the image plane to expand the light beam and image the light beam simultaneously on a substantially linear portion of the photosensitive surface, wherein the imaging optics comprises (i) a fourier transform lens configured such that light from each beam component converges around a corresponding point of a back focal plane of the fourier transform lens, (ii) an optical component for inverse fourier transformation of light from the back focal plane to the image plane, wherein the optical component for inverse fourier transformation of light comprises an inverse fourier transform mirror, and (iii) a filter positioned at the back focal plane; and
a drum covered by the photosensitive surface, wherein the drum is configured to rotate the photosensitive surface such that the substantially linear portion imaged by the light beam is scanned across an area of the photosensitive surface.
1. A printing system comprising:
a laser source to generate a light beam;
a linear diffractive spatial light modulator assembly to diffract light from the laser source according to an applied electric field;
illumination optics for focusing the light beam onto the spatial light modulator assembly;
an image plane having a photosensitive surface;
imaging optics disposed in a light path between the spatial light modulator assembly and the image plane to expand the light beam and image the light beam simultaneously on a substantially linear portion of the photosensitive surface, wherein the imaging optics comprises (i) a fourier transform lens configured such that light from each beam component converges around a corresponding point of a back focal plane of the fourier transform lens, and (ii) an optical component for inverse fourier transformation of light from the back focal plane to the image plane; and
a drum covered by the photosensitive surface, wherein the drum is configured to rotate the photosensitive surface such that the substantially linear portion imaged by the light beam is scanned across an area of the photosensitive surface,
wherein the laser source comprises an array of a plurality of laser emitters, and the illumination and imaging optics are configured to compose a corresponding plurality of images at the image plane,
wherein the illumination optics comprises a plurality of lenses configured to stitch together beam components from the plurality of emitters to form the light beam, lenses configured to magnify each said beam component, and an optical component to bend said beam components so that said beam components are incident to the spatial light modulator assembly at different incident angles.
2. The printing system according to
3. The printing system according to
4. The printing system according to
5. The printing system according to
6. The printing system according to
7. The printing system according to
8. The printing system according to
11. The method according to
12. The method according to
13. The method according to
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The present application claims the benefit of U.S. provisional application No. 60/528,529, entitled “Apparatus and Method for Laser Printing Using a Spatial Light Modulator,” filed Dec. 10, 2003, by inventors Clinton B. Carlisle, Jahja I. Trisnadi, David T. Amm, and Anthony A. Abdilla, the disclosure of which is hereby incorporated by reference.
The present invention is directed generally to a laser printer utilizing spatial light modulators, and more particularly to a laser printer utilizing a linear diffractive spatial light modulator.
Conventional laser printers use mechanical scanners to scan a laser spot onto a photosensitive or photoconductive surface. The photosensitive or photoconductive surface may be, for example, on a drum 108. Typical laser printers have scanning optics that include a laser 102 for generating laser light, a multifaceted mirror or scanner 104 that spins at high speed for scanning laser light. A layout for a conventional laser printer 100 is shown in
However, the limitations of such an approach are equally evident. Scanners require a predetermined time to spin up to operating speed prior to printing a first page, and the spinning speed inherently limits how fast the scanner can scan. The mechanical nature of this scanning mechanism is thus disadvantageous and also leads to increased operating noise and maintenance costs.
Additionally, while conventional scanning optics 106 can be satisfactorily used in a wide variety of printing applications, there are emerging applications that require even higher pixel resolutions than can be provided by the architecture described above.
Accordingly, there is a need for a linear spatial light modulator that exhibits the following characteristics: good analog gray-scale capability, high modulation speed, high diffraction efficiency, and a large number of “channel” count (1000-10,000). There is a further need for a method of manufacturing such a spatial light modulator that is simple, cost-effective, and tolerant of process variations.
The present disclosure provides a solution to these and other problems, and offers further advantages over conventional laser printers.
In one aspect, the present invention is directed to a printing system having a linear diffractive spatial light modulator (LDSLM) assembly that diffracts light from a laser source according to or under the influence of an applied electric field applied to the LDSLM assembly. Generally, the printing system further includes illumination optics for focusing the light beam onto the LDSLM assembly, an image plane having an array of photosensitive elements or a photosensitive surface, and imaging optics disposed in a light path between the spatial light modulator assembly and the image plane to expand the light beam and impinge the light beam simultaneously on a substantially linear portion of the photosensitive surface.
In one embodiment, the LDSLM assembly includes a linear array of diffractive MEMS elements. For example, each of the diffractive MEMS elements can include a number of deflectable ribbons having a light reflective planar surface. In one version of this embodiment, the LDSLM assembly includes a linear array of diffractive MEMS elements grouped in a number of pixels, and each of the diffractive MEMS elements in a single pixel share a common ribbon structure.
In another embodiment, each of the diffractive MEMS elements further include a substrate on which the ribbons and drive electronics to apply an electric field to the ribbons is integrally formed. Preferably, the linear array of diffractive MEMS elements including the ribbons and drive electronics are integrally formed on a single substrate.
In other embodiments, the LDSM assembly can include two or more linear arrays of diffractive MEMS elements, and the laser source can include an array of multiple lasers or laser emitters.
These and various other features and advantages of the present invention will be apparent upon reading of the following detailed description in conjunction with the accompanying drawings and the appended claims provided below, where:
The use of the same reference label in different drawings indicates the same or like components. Drawings are not necessarily to scale unless otherwise noted.
The present invention is directed to a novel printing system having a linear diffractive spatial light modulator (LDSLM) assembly that diffracts light from a laser source according to or under the influence of an applied electric field applied to the LDSLM assembly.
An architecture 200 for a laser printer according to an embodiment of the present invention is shown in
Referring to
The laser source 202 can include a number of lasers or laser emitters, such as low-power diode lasers, each powered from a common power supply (not shown) in a CW (Continuous Wave) operation.
The illumination optics 204 can comprise a number of elements including lens integrators, mirrors and prisms, designed to transfer light from the laser source 202 to the LDSLM 205 such that a line of a specified size is illuminated at the LDSLM 205. In particular, the illumination optics 204 are adapted to illuminated a swath covering substantially the full width of the LDSLM 205.
The imaging optics can comprise magnification elements, such as the FT lens 206 and mirror 208, and filter elements, such as the FT filter 207, designed to transfer light from the LDSLM 205 to the drum 210 such that the photoconductive layer located on the drum 210 is illuminated across a swath covering substantially the full width of the drum 210.
Some embodiments of diffractive MEMS elements and pixel structures for the diffractive MEMS elements of the LDSLM 205 according to the present invention will now be described with reference to
Referring to
In an alternative embodiment, shown in
A ribbon and a gap pair 310 in
In a preferred embodiment, shown in
Referring to
The LDLSM 205 can be operated in zero-order or first-order modes. In the zero-order mode, the 0th-order diffraction (or reflection) is collected and modulation is obtained by diffracting the light away into first and higher orders. In first-order mode, it is the modulated 1st-order diffractions that are collected. However, since the LDSLM period is likely to be just a few wavelengths, the diffraction angle is very large. Therefore, zero-order operation is more desirable. The distance between the ribbon reflective layer and the bottom reflective layer is H. If H=odd×λ/4, the LDSLM is normally OFF, i.e. the un-activated state is diffracting (so light is discarded), which corresponds to a dark pixel in the 0th order mode. If H=even×λ/4, the LDSLM is normally ON, i.e. the un-activated state is specular, which corresponds to a bright pixel in the 0th order mode. Since the ribbon snaps down to the substrate if the deflection exceeds H/3, the smallest even (odd) multiplier is four (five). Alternatively, a height margin δ is added to permit uniformity calibration (e.g. 5λ/4+δ).
As an example, Λ=1 μm, N=3, periods/pixel, giving P=3 μm. The slot width as well as the inter-ribbon gap=0.5 μm. The ribbon width is W=2.5 μm. For λ=0.5 μm, a normally off device has a H=0.625 μm (odd=5) or H=0.875 μm (odd=7). Thus, for 10,000 pixels the die is only 30 mm long.
In yet another aspect of the present invention, the inventive architecture can be scaled to even higher pixel count by employing two or more LDSLMs. Two or more LDSLMs can be employed in two symmetrically offset positions as shown in
Example
For a diffractive-MEMS-based laser printer using the product architecture 200 illustrated schematically in
For this example calculation on the printing speeds attainable by the linear, diffractive MEMS-based laser printer, the following system parameters are defined as:
Printing speed=R [area/time]
Photoconductor sensitivity=S [exposure fluence=energy/area]
Optical throughput=η
The desired laser power=P [energy/time]
Laser wavelength=λ=780 nm (GaAs diode laser)
For example, R=1 letter-size paper/sec=30×20 cm2/s (excluding overhead, such as paper feeding), S=1.5 μJ/cm2 at 800 nm wavelength, η=30%, then P=3 mW.
Resolution=r
Paper width=Wpaper
Paper length=Lpaper
The paper linear speed (hence the drum linear speed) is
which, in this example, is 30 cm/sec, and the LDSLM modulation speed is
so at r=12.5 μm (2000 dpi), LDSLM pixel speed=(30 cm/sec)/(12.5 μm)=24 kHz.
The desired number of LDSLM pixels NGLV is
so at r=12.5 μm (2000 dpi) and Wpaper=8″=200 mm, NGLV=16000.
To maintain a reasonable die size for that many pixels, the pixel size w is preferably small (a few microns). The optics magnification will then be
With the slotted ribbon diffractive LDSLM described earlier, 3 μm pixels become feasible. In this case, the optics magnification is M=12.5/3=4.17×
The desired illumination-optics speed is
which in the example is NAillum=0.78/3=0.27 (F/1.8).
The desired imaging optics speed is
which in the example is NAimg=0.78/12.5=0.064 (F/7.8) with about 50 μm depth-of-focus.
Results:
Thus, the imaging system of the present invention provides increased resolution and efficiency over conventional laser printing architectures. For example, a system designed in accordance with the embodiments described above is capable of a resolution of 2000 dpi (dots per inch) at a printing speed of as much as about 2000 pages per minute (ppm). However, it will be understood that the actually printing speed is limited by non-LDSLM factors to about 60 letter-size ppm. With 1.5 μJ/cm2 photoconductor sensitivity and with 30% laser-to-drum efficiency, the desired laser power is 3 mW. In one preferred embodiment, the LDSLM 205 has 16000 pixels, each of which is 3 μm wide, and the desired LDSLM pixel modulation speed to produce 1 letter-size/sec is 24 kHz. The illumination NA is 0.26, and the imaging NA is 0.064. The desired data flow to the printing head=24×103×16,000×8 bits (for gray-scale)=3 Gbits/sec, for printing 60 pages/minute
This example calculation demonstrates that a linear, diffractive MEMS LDSLM 205 can enable very high-speed laser printing with simple diode lasers and optics and standard-sensitivity photoconductive write drums. Furthermore, the modulation speed of the LDSLM 205 can easily be increased by more than an order magnitude beyond the value of 24 kHz cited in the example calculation above. The data flow requirements between the PC and the printer are high (3 Gbits/sec) but certainly attainable with state-of-the-art technologies. It is additionally interesting to note that the maximum printing speed allowed by the LDSLM (˜1 MHz) is ˜2000 pages/min.
Multiple Laser-Beam Architecture
The printing speeds calculated can all be achieved with a printer capable of 8-bit gray-scale exposure using the low-power diode laser in CW operation. Therefore, the present invention is directed to a printing system having a LDSLM assembly and a multiple laser-beam optical architecture to effectively increase the laser-printer resolution by sequential tiling of NL images (on the printer photoconductive drum) from a single diffractive MEMS spatial light modulator illuminated by NL laser sources.
Two-Beam Illumination
For simplicity, a system 700 having two laser illumination is illustrated in
1.1 Two-Beam Imaging: LDSLM-to-Drum
Given an LDSLM 202 of length h and the total length of the image on the drum 704 of H, the desired magnification is
The magnification is realized by the choice of the FT and FT−1 focal lengths, since M=f2/f1. It can be seen that there is only one parameter left to fix the system, which we will take to be the imaging optics length L≡2 f1+2 f2. Solving, we obtain:
The FT lenses (206 and 208) are placed at the location where the two beams start to separate. The desired illumination incident angle θ1 is
The incident angle on the drum±θ2 is
Example #1
Given h=25 mm (5000 pixels, say), H=8″ (total of 10,000 pixels to meet 1200 dpi), and L=400 mm, we find that
M=H/2 h=4.
f1=40 mm, f2=160 mm
θ1=17.4°, θ2=8.9° (L can be increased if these angles are too large)
The angles are not very small and it may raise some concerns (especially since the imaging is not telecentric). The dependencies of θ1 and θ2 on L is shown in
1.2 Two-Beam Illumination: Laser-to-LDSLM
A two-beam laser to LDSLM system is illustrated in
Assume a light source 202 comprising two point sources 902, such as two laser emitters or emitters on a single substrate or GaAs die. The two point sources 902 may be configured to be apart by a distance d. What is needed is an illumination system with two parameters to match the illumination width h and the incident angles of the two beams θ1 on the LDSLM 205. The example in
In this implementation, the illumination optics 204 is configured to stitch 904 the beams, magnify 906 each beam from d to h, and to bend 908 the two beams to plus θ1 and minus θ1. To bend the beam by θ1, the prism angle (see
α=sin−1(n sin α)−θ1 (5)
Example #2
Recall that from last the last example h=25 mm and θ1=17.4°:
If d=250 μm, the desired magnification is 100×. Further, the prism (index n=1.5) preferably has an α=9.25°.
Conclusion:
Thus, the imaging system of the present invention is uniquely defined by the LDSLM length h, the image width on the drum H and the imaging optics length L. The illumination system is uniquely defined by the laser emitter spacing d, the beam width h and the incident angles±θ1. Preferably, the illumination optics are simple, compact, robust, and cheap, for example from molded polymers or plastics.
A complete optics diagram of an embodiment of a dual-laser printer architecture, including both the illumination 900 and imaging 700 systems is shown in
2. Generalization to Multi-Beam Illumination
In yet another alternative embodiment, not shown, the illumination system can be a multi-beam illumination system including NL lasers, where NL is greater than two.
2.1 Multi-Beam Imaging
With NL lasers, a GLV of length h, and an image total length H on the drum, the required magnification is
As before, there is only one parameter left to fix the system, which we will take to be the imaging optics length L=2f1+2f2. Solving, we obtain:
The largest illumination incident angle θ1 is
and the incident angle on the drum±θ2 is:
Example:
Use NL=3 with h=25 mm (5000 pixels, say), H=12″ (total of 15 k pixels to meet 1200 dpi), and L=400 mm, we find that
M=H/3 h=4×, f1=36.4 mm, f2=97.0 mm, θ1=34.5°, and θ2=27.3°. Increasing the optics train length to L=1 meter helps to bring the parameters down to f1=90.9 mm, f2=242.4 mm, θ1=15.4°, and θ2=11.7°.
2.2 Multi-Beam Illumination
The design of this multi-beam illumination system is analogous to the two laser system described previously, but with the triangular prism replaced by a multi-faceted prism to deliver NL beams to the GLV with various incident angles between −θ1, and +θ1.
Conclusion:
Thus, the multi-beam illumination system of the present invention effectively increases printer resolution by sequential tiling of NL images (on the printer photoconductive drum) from a single LDSLM illuminated by NL laser sources. The illumination system is uniquely defined by the number of lasers NL, the laser emitter spacing d, the GLV length h, the image width on the drum H and the imaging optics length L.
The foregoing description of specific embodiments and examples of the invention have been presented for the purpose of illustration and description, and although the invention has been described and illustrated by certain of the preceding examples, it is not to be construed as being limited thereby. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications, improvements and variations within the scope of the invention are possible in light of the above teaching. It is intended that the scope of the invention encompass the generic area as herein disclosed, and by the claims appended hereto and their equivalents.
Amm, David T., Trisnadi, Jahja I., Carlisle, Clinton B., Abdilla, Anthony A.
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